There are numerous applications in the electrohydraulic control field in which it is desired to control motion and/or pressure at an actuator system and load. In a typical machine for bending tube stock, for example, a bending head includes a mandrel and an actuated die for bending tube stock around the mandrel. During the bending operation, the tube stock is clamped or gripped upstream of the bending head, and is urged toward the bending head during the bending operation to prevent thinning of the tube wall. The clamp is allowed to slip lengthwise of the tube stock, but it is desirable to push the stock into the bending head with pressure that is precisely controlled as a function of motion of the bend actuator and die.
In tube bending machines of the described character, pressure applied by a boost actuator to the tube clamp has been measured, and a pressure relief valve has been modulated to obtain a desired profile of pressure versus time. However, it has not heretofore been attempted to control velocity of slip of the clamping mechanism along the tube stock, or to control lengthwise pressure applied to the tube stock as a function of such velocity. Consequently, control systems heretofore proposed have not obtained desired quality control of the bending operation, particularly as applied to thinning of the tube wall during bending.
It is therefore a general object of the present invention to provide an electrohydraulic actuator system that obtains enhanced and precise control of both pressure and motion at the actuator and load. Another object of the present invention is to provide a system of the described character that embodies state-of-the-art electronic control capability, and yet is easy and economical to implement both in new system construction and in retrofit of existing systems.
Another and more specific object of the present invention is to provide an electrohydraulic system for controlling pressure and velocity at an actuator load, such as at the boost cylinder of a tube bending machine, at a precise programmable function. A related object of the invention is to provide an electrohydraulic system for bending tube stock that features enhanced control of the boost cylinder for urging the tube stock lengthwise into the bend head to reduce thinning during the bending operation.